Gyroscope and fabrication method thereof

ABSTRACT

A gyroscope which comprises: a driving fixed electrode ( 26 ) being fixed; a driving displacement electrode ( 24 ) being opposite to the driving fixed electrode, and being able to be displaced in a first direction; an inertial mass ( 23 ) being connected to the driving displacement electrode ( 24 ), being displaced in the first direction according to the first directional displacement of the driving displacement electrode ( 24 ), and being displaced in a second direction when an angular rate is applied; a sensing displacement electrode ( 22 ) being connected to the inertial mass ( 23 ), and being able to be displaced in the second direction according to the second directional displacement of the inertial mass ( 23 ); and a sensing fixed electrode ( 25 ) being opposite to the sensing displacement electrode and being fixed. The driving displacement electrode ( 24 ) is supported by a folded spring ( 31 ) movable in the first direction, and the sensing displacement electrode is supported by a folded spring ( 32 ) movable in the second direction.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a micromachined gyroscope and afabrication method thereof.

[0003] (b) Description of the Related Art

[0004] Micro inertial sensors are one application among variousapplication fields of micromachining techniques. Since micromachinedinertial sensors made of silicon are inexpensive, mass-producible, andcan be integrated, researches on commercial inertial sensors have beenactively performed during the past decade. However, while actualcommercial gyroscopes were fabricated and produced a few years ago,micromachined gyroscopes have not been commercialized yet. For theircommercialization, it is required to discriminate low output values fromnoise, selectively sense the low output values and obtain highsensitivity together with wide operation ranges. It is further requiredto use a process that is perfectly compatible with that of conventionalsemiconductors or to use a simpler process to implement inexpensiveelements. Also, the elements' reliabilities and high yield must beguaranteed.

[0005] In 1998, Analog Devices Inc. (ADI) used the surfacemicromachining process to develop integrated gyroscope prototypesoperating in a vacuum environment with the goal of producing thegyroscopes for $30. However, since gyroscopes using piezo-electricelements cost $15 for autos and $5 for cameras, a new approach wasnecessary to compete with these expensive devices. Accordingly, ADIapproached development of inexpensive gyroscopes through minimization ofelement size and packaging processes. To minimize the element size, acircuit portion that occupies the bulk of the element was reduced. Theproblems caused by reduction of the circuit portion were solved byenlarging the mechanical portion of the gyroscope, thereby reducing theoverall size of the element. In addition, when the mechanical portion isenlarged, the element can operate at a low Q value, and thereby twoobjectives of commercialization are satisfied: a small-sized element andelimination of the vacuum sealing. The vacuum sealing processconstitutes 80% of the cost, compared with general IC packagingprocesses at 50%. It should be required to eliminate the vacuum sealingprocess for cost down of the gyroscopes.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a gyroscopeoperable in atmospheric pressure.

[0007] It is another object of the present invention to provide agyroscope that has a large driving displacement at high frequencies.

[0008] It is still another object of the present invention to provide agyroscope that removes mechanical interference between the driving modeand the sensing mode.

[0009] It is further another object of the present invention to providea gyroscope that produces high sensitivity without frequency tuning.

[0010] It is further another object of the present invention to simplifya method for fabricating gyroscopes.

[0011] In order to achieve the objects, a driving displacement electrodeand a sensing displacement electrode are connected through an inertialmass and a folded spring so as to remove mechanical interference betweenthe driving displacement electrode and the sensing displacementelectrode.

[0012] In one aspect of the present invention, a gyroscope comprises: adriving fixed electrode being fixed; a driving displacement electrodebeing opposite to the driving fixed electrode, and being able to bedisplaced in the first direction; an inertial mass being connected tothe driving displacement electrode, being displaced in the firstdirection according to the first directional displacement of the drivingdisplacement electrode, and being displaced in the second direction whenan angular rate is applied; a sensing displacement electrode beingconnected to the inertial mass, and being able to be displaced in thesecond direction according to the second directional displacement of theinertial mass; and a sensing fixed electrode being opposite to thesensing displacement electrode and being fixed.

[0013] The driving displacement electrode is supported by a foldedspring movable in the first direction, and the sensing displacementelectrode is supported by a folded spring movable in the seconddirection.

[0014] The driving displacement electrode and the inertial mass can bemovable in the second direction and are connected by a folded springhaving no fixture shaft, and the sensing displacement electrode and theinertial mass can be movable in the first direction and are connected bya folded spring having no fixture shaft.

[0015] Two sensing displacement electrodes are provided on two sides ofthe inertial mass, and the gyroscope further comprises an edge gimbalfor connecting the two sensing displacement electrodes.

[0016] A cavity is provided in the center of the inertial mass, and thegyroscope further comprises a displacement limit shaft provided to thecenter of the cavity and being fixed.

[0017] The gyroscope further comprises tuning electrodes, eachsymmetrically formed on both sides of the sensing displacementelectrode, functioning as electrical springs to vary the resonancefrequency of the sensing displacement electrode and control sensingsensitivity.

[0018] In another aspect of the present invention, a method forfabricating a gyroscope comprises: (a) performing anodic bonding on asilicon substrate and a glass substrate; (b) etching and polishing thesilicon substrate to be a predetermined thickness; (c) forming ametallic layer on the silicon substrate; (d) performing photolithographyon the metallic layer and the silicon substrate to form a siliconstructure having a gyroscope pattern; (e) etching the glass substrate,and separating remaining portions of the silicon structure except afixture shaft from the glass substrate to form a gyroscope structure;and (f) performing flip chip bonding on the gyroscope structure toconnect it to an external circuit.

[0019] The method further comprises dicing the silicon substrate and theglass substrate and separating them into respective elements between (d)and (e).

[0020] The metallic layer formed in (c) is a double layer of Cr and Au,and the glass substrate is etched using an HF solution in (e).

[0021] The depth for etching the glass substrate using the HF solutionis set to be greater than 10 μm so as to minimize air damping and tooperate the gyroscope in atmospheric pressure.

[0022] The silicon substrate is etched using a KOH aqueous solution of36 wt. % at 80° C. in (b).

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate an embodiment of theinvention, and, together with the description, serve to explain theprinciples of the invention:

[0024]FIG. 1 shows a perspective view of a gyroscope according to apreferred embodiment of the present invention;

[0025]FIG. 2 shows a floor plan of a gyroscope according to a preferredembodiment of the present invention;

[0026]FIG. 3 shows a magnified portion of FIG. 1;

[0027]FIG. 4 shows a partial SEM(scanning Electron Microscope)photograph of a gyroscope fabricated according to a preferred embodimentof the present invention;

[0028]FIG. 5 shows a magnified view of a driving or sensing spring of agyroscope according to a preferred embodiment of the present invention;

[0029]FIG. 6 shows an SEM photograph of a driving or sensing spring of agyroscope fabricated according to a preferred embodiment of the presentinvention;

[0030]FIG. 7 shows a concept view of a driving or sensing connectionspring of a gyroscope according to a preferred embodiment of the presentinvention;

[0031]FIG. 8 shows a graph of resonance driving and sensing displacementaccording to a displacement limiter; and

[0032] FIGS. 9(a) to 9(h) show a process for fabricating a gyroscopeaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] In the following detailed description, only the preferredembodiment of the invention has been shown and described, simply by wayof illustration of the best mode contemplated by the inventor(s) ofcarrying out the invention. As will be realized, the invention iscapable of modification in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawings and descriptionare to be regarded as illustrative in nature, and not restrictive.

[0034]FIG. 1 shows a perspective view of a gyroscope according to apreferred embodiment of the present invention, FIG. 2 shows a floor planof a gyroscope according to a preferred embodiment of the presentinvention, FIG. 3 shows a magnified portion of FIG. 1, and FIG. 4 showsa partial SEM photograph of a gyroscope fabricated according to apreferred embodiment of the present invention.

[0035] The gyroscope according to the preferred embodiment of thepresent invention comprises a glass substrate 10 and a silicon structureformed thereon. Supporting columns 11, 12, and 13 for supporting thesilicon structure and fixing a fixture shaft of the silicon structureare formed on the glass substrate 10.

[0036] The silicon structure comprises a driver, sensor, a plurality ofsprings, a tuning electrode 34, and a displacement limit shaft 33. Thedriver comprises a driving fixing electrode 26, a driving displacementelectrode 24, and an inertial mass 23. The sensor comprises a sensingfixing electrode 25, a sensing displacement electrode 22, and an edgegimbal 21. The springs comprise driving springs 28 and 29 for supportingand allowing vibration of the driving displacement electrode 24, sensingsprings 27 and 30 for supporting and allowing vibration of the sensingdisplacement electrode 22 and the edge gimbal 21, a driving connectionspring 31 for connecting the driving displacement electrode 24 with theinertial mass 23, and a sensing connection spring 32 for connecting thesensing displacement electrode 22 with the inertial mass 23. The drivingsprings 28 and 29 are classified as an external driving spring 28 and aninternal driving spring 29 respectively, and the sensing springs 27 and30 are respectively an external sensing spring 27 and an internalsensing spring 30.

[0037] A further detailed description of structures and function ofthese components will now be provided.

[0038] The driving fixing electrode 26 of the driver is fixed by thesupporting column 12 of the glass substrate 10 and has a spaced toothedportion. The driving displacement electrode 24 is supported by theexternal and internal driving springs 28 and 29, is movable only in theeast to west direction. (refer to the compass direction of FIGS. 1 and2), and also has a spaced toothed portion. The teeth of the spacedtoothed portion of the driving fixing electrode 26 and those of thedriving displacement electrode 24 are interspersed with each other. Theexternal and internal driving springs 28 and 29 are fixed by thesupporting column 13 of the glass substrate 10, and since they are platesprings and are provided in the south to north direction, they aremovable only in the east to west direction. Since the inertial mass 23is connected to the driving displacement electrode 24 through thedriving connection spring 31, when the driving displacement electrode 24is vibrated in the east to west direction, the inertial mass 23 isvibrated together with the driving displacement electrode 24. In thisinstance, since the driving connection spring 31 has no fixing shaft andcan be movable only in the south to north direction, it delivers east towest vibration of the driving displacement electrode 24 to the inertialmass 23.

[0039] Next, the sensing fixing electrode 25 of the sensor, fixed by thesupporting column 12 of the glass substrate 10, has a spaced toothedportion. The sensing displacement electrode 22 is supported by theexternal and internal sensing springs 27 and 30, is movable only in thesouth to north direction (refer to the compass direction of FIGS. 1 and2), and also has a spaced toothed portion. The teeth of the spacedtoothed portion of the sensing fixing electrode 25 and those of thesensing displacement electrode 22 are interspersed with each other. Theexternal and internal sensing springs 27 and 30 are fixed by thesupporting column 13 of the glass substrate 10, and since they are platesprings and are provided in the east to west direction, they are movableonly in the south to north direction. The sensing displacement electrode22 is connected to the inertial mass 23 through the sensing connectionspring 32, and accordingly, when the inertial mass 23 is vibrated in thesouth to north direction, the sensing displacement electrode 22 isvibrated together with the inertial mass 23. In this instance, since thesensing connection spring 32 has no fixing shaft and can be movable onlyin the east to west direction, the sensing connection spring 32 deliversthe south to north directional vibration of the inertial mass 23 to thesensing displacement electrode 22 as it is. The edge gimbal 21 totallysurrounds the driver and the sensor, and connects the sensingdisplacement electrodes 22 positioned on the south and north sides ofthe inertial mass 23. Therefore, the sensing displacement electrodes 22on both sides are operated in the identical direction and with theidentical displacement.

[0040] Each tuning electrode 34, of a total of four, is formed on theeast and west sides of the two south to north sensing displacementelectrodes 22, and is fixed by a supporting column 11 of the glasssubstrate 10. A spaced toothed portion is formed on the tuningelectrode's surface facing the sensing displacement electrodes 22, and aspaced tooth portion is also formed on the sensing displacementelectrode's surface facing the tuning electrode 34. The teeth of thesensing displacement electrodes 22 face those of the tuning electrode34, and the spaces of the sensing displacement electrodes 22 face thoseof the tuning electrode 34.

[0041] The displacement limit shaft 33, formed in the center of a cavityformed in the center of the inertial mass 23, prevents the inertial mass23 from being displaced over a predetermined level, and it is fixed by asupporting column (not illustrated) of the glass substrate 10. When theinertial mass 23 is stopped, the displacement limit shaft 33 isseparated on one side from the inertial mass 23 by a predetermined gap.

[0042] A method for driving the above-structured gyroscope and a sensingmethod will now be described.

[0043] When power is supplied to the driving fixing electrode 26, thedriving displacement electrode 24 is electrostatically driven accordingto the frequency of the power supplied to the driving fixing electrode26, and it is vibrated in the east to west direction. The inertial mass23 is also vibrated in the same manner of the driving displacementelectrode 24. When torque is applied to the gyroscope under this state,the inertial mass 23 receives the south to north directional force andis vibrated in the south to north direction. The south to northdirectional vibration of the inertial mass 23 is delivered to thesensing displacement electrode 22 as it is, and the sensing displacementelectrodes 22 are vibrated in the same manner. When the sensingdisplacement electrodes 22 are vibrated, the capacitance generatedbetween the sensing fixing electrode 25 and the sensing displacementelectrode 22 is varied, and an angular rate is calculated by sensing thevariation. In this instance, the tuning electrode 34 functions as anelectric spring, and it varies the resonance frequency of the sensingfixing electrode 25 to adjust sensing sensitivity. The displacementlimit shaft 33 limits driving displacement of the inertial mass 23within a uniform value so as to maintain a uniform displacement in awide frequency range. This will be described subsequently.

[0044] With reference to FIGS. 5 and 6, the structure of the driving orsensing springs 27, 28, 29, and 30 applied to the preferred embodimentof the present invention will be described in detail.

[0045]FIG. 5 shows a magnified view of a driving or sensing spring of agyroscope according to a preferred embodiment of the present invention,and FIG. 6 shows an SEM photograph of a driving or sensing spring of agyroscope fabricated according to a preferred embodiment of the presentinvention.

[0046] The driving or sensing spring comprises a fixture shaft 1, aconnector 2, an internal plate 3 and an external plate 4. The fixtureshaft 1 is fixed by the supporting column 13 of the glass substrate 10,and the connector 2 connects the internal plate 3 with the externalplate 4. The internal plate 3 connects between the fixture shaft 1 andthe connector 2, and the external plate 4 connects between the connector2 and the driving or sensing displacement electrode (which is determinedaccording to a driving spring or a sensing spring). The spring of thisstructure is referred to as a folded spring.

[0047] Referring to FIG. 5, the thin layer provided on the siliconstructure is a metallic layer. This metallic layer is formed to performflip chip bonding on the gyroscope. FIGS. 1 to 4 omit this illustration.

[0048] Referring to FIG. 6, the connector 2 and the cavity formed aroundthe structure are used for injecting etchant when etching the glasssubstrate so as to raise the structure in the air in the fabricationprocess. Therefore, the cavities are formed on all portions of thestructure except for the fixture shaft and the narrow spring plates 3and 4.

[0049] With reference to FIG. 7, the structures of the driving orsensing connection spring 31 or 32 applied to the preferred embodimentof the present invention will now be described in detail.

[0050]FIG. 7 shows a concept view of a driving or sensing connectionspring of a gyroscope according to a preferred embodiment of the presentinvention.

[0051] The connection spring comprises a connector 2, an internal plate3, and an external plate 4, and has no fixture shaft. The connector 2connects the internal plate 3 with the external plate 4, and theinternal plate 3 is connected to either a driving displacement electrodeor a sensing displacement electrode (which is determined according to adriving connection spring or a sensing connection spring), and theexternal plate 4 is connected to the inertial mass 23.

[0052] Next, functions of the displacement limiter will be described.

[0053]FIG. 8 shows a graph of resonance driving and sensing displacementaccording to a displacement limiter.

[0054] In the case of a resonant structure of a high Q value, sincedisplacement variations according to degrees of vacuum and variations offrequency are very large and corresponding outputs of the structure areaccordingly varied, it has a problem in that the output performance isvery sensitive to external noise (the degrees of vacuum or variations offrequency). To overcome this problem, the present gyroscope provides amechanical displacement limiter for limiting driving displacement to thecenter portion of the inertial mass, so that the displacement of theinertial mass and the driver is maintained at a uniform value anduniform displacement is maintained in a wide frequency range. Also, asshown in FIG. 8, while in the case of conventional gyroscopes theresonance frequency needs to be tuned in order to prevent lowering ofsensitivity when the driving and sensing resonance frequencies aredifferent because of process errors, in the case of the presentgyroscope that adopts a displacement limiter, the driving displacementis uniformly maintained in the frequency range of wide drivingdisplacement and the driving displacement is uniformly maintained in thesensing resonance frequency range, and hence, the gyroscope can beoperated in the sensing resonance frequency without additional tuning.

[0055] A method for fabricating the above-structured gyroscope will nowbe described.

[0056] FIGS. 9(a) to 9(h) show a process for fabricating a gyroscopeaccording to a preferred embodiment of the present invention.

[0057] As shown in FIG. 9(a), 800 volts (V) at 380° C. are supplied to alow resistive silicon wafer 20 and a glass substrate 10 such as Pyrex#7740 for two hours to perform anodic bonding.

[0058] As shown in FIG. 9(b), the silicon wafer 20 is etched using a KOHaqueous solution. After the etching, the thickness of the silicon wafer20 becomes about 70 μm, which represents the addition of the finalthickness of the silicon structure that is 50 μm and an extra thicknessof 20 μm for generating a mirror surface through chemical mechanicalpolishing (CMP.) In this instance, the etchant is a 36 wt. % KOH aqueoussolution, used at 80° C., and the corresponding etching speed is about0.93 μm/min. After the KOH etching, hillocks and pit holes are formed onthe silicon surface.

[0059] As shown in FIG. 9(c), the surface of the silicon wafer 20 isprocessed to be a mirror surface through the CMP.

[0060] As shown in FIG. 9(d), in order to form an electrode for flipchip bonding, Cr and Au are deposited in the respective thickness of 200and 300 Åto form a Cr/Au layer 40.

[0061] In FIG. 9(e), an oxidized layer (not illustrated) is deposited onthe Cr/Au layer 40, photoresist is coated thereon, and a photo processis performed to form a photoresist pattern 50 for defining a siliconstructure pattern. In this instance, the photoresist pattern includes apattern for forming an etchant penetration cavity needed for etching theglass substrate and floating the silicon structure.

[0062] As shown in FIG. 9(f), the oxidized layer is etched using thephotoresist pattern 50 as an etching mask, and reactive ion etching(RIE) is executed using the photoresist pattern 50 and the oxidizedlayer as etching masks to form a silicon structure pattern. The siliconwafer 20 and the glass substrate 10 on which the silicon structurepattern is formed are then diced and split into cells.

[0063] As shown in FIG. 9(g), in order to separate the silicon structure20 from the glass substrate 10, the glass substrate 10 is etched in a49% HF solution. In this instance, as described above, cavities areformed on all portions of the silicon structure except the fixture shaftand the plate of the spring so that the etchant may reach the glasssubstrate 10. Here, by floating the structure above the glass substratewith a sufficient distance to minimize air damping, gyroscopes thatoperate in atmospheric pressure can be fabricated, and when glassetching using the HF solution, a structure that floats above the glasssubstrate by at least 10 μm can be fabricated. In this instance, thestructure must be floated above the glass substrate with full distanceto minimize the air damping. By etching the glass substrate 10 for about8 minutes, the silicon structure 20 can float above the glass substrate10 with the gap of about 50 μm.

[0064] As shown in FIG. 9(h), finally, a gyroscope structure 200comprising the glass substrate 10, the silicon structure, and the Cr/Aulayer 40 is flip-chip-bonded on an electrode structure for wiring on aprinted circuit board (PCB) 100. In this instance, a bonder 300 iscontacted with fixture shaft portions of the gyroscope structure 200.

[0065] In the above, the photoresist and the oxidized layer are used asetching masks when patterning the silicon structure, and further, it ispossible to only use the photoresist.

[0066] Features of the gyroscope according to the present invention willnow be described.

[0067] The gyroscope is fabricated based on very simple processes, andthe photo etching process for fabricating the gyroscope structure in thewhole process is executed once. The fabricated gyroscope has only onestructure layer, and flip chip bonding is finally performed so that thegyroscope is connected to a circuit.

[0068] The driver and the sensor of the gyroscope structure adopt acombed structure, and are designed to minimize air damping by enlargingthe gap from the ground surface and preserve a high Q value inatmospheric pressure so that a Q value equation that has not beenconsidered in the design of conventional gyroscopes may be determined,and the Q value of the structure may be predicted based on thearrangement so as to maximize mechanical sensitivity. In this instance,Couette flow, Stokes flow, and squeeze damping are considered to predictthe Q value.

[0069] Also, to obtain the maximum sensitivity, a maximal electrodestructure is integrated in the given gyroscope region so as to have alarge driving displacement of as much as 10 μm, and a correspondingmechanical structure is designed to minimize mechanical interference ofthe gyroscope and be insensitive to external noise. The driving andsensing frequencies of the structure are designed to be greater than 5kHz in order to remove influence of external noise, and the driving andsensing frequencies are designed not to be matched but to be separatedby about 50 Hz so as to enlarge the bandwidth. The driving and sensingfrequencies are predicted to be 7,088 Hz and 7,132 Hz, respectively.

[0070] Further, in the case of the gyroscope according to the presentinvention, a displacement limiter that artificially limits thedisplacement is attached to the gyroscope, which maintains the drivingdisplacement in the wide frequency range to widen the bandwidth andreduces necessity of tuning.

[0071] The gyroscope structure has a form basically identical with atwo-dimensional stage, and a driving mode and a sensing mode exist inthe identical plane. Since the sensitivity of the gyroscope increases asthe inertial mass becomes greater, the whole mass of the gyroscope mustbe large. Also, the number of combed electrode structures in thestructure is maximized in order to maximize the driving and sensingsensitivities, and edge gimbals are adopted in order to minimizemechanical interference. The size of the gyroscope structure is 8×8 mm²,and the whole size, considering an external frame for hermetic sealing,is 10×10 mm². Single crystal silicon having excellent mechanicalfeatures is used as described above, and the thickness of the structureis designed to be 50 μm. Also, the gyroscope structure is floated abovethe glass substrate by 50 μm so as to reduce damping.

[0072] As described above, in the case of a structure that vibrates inthe horizontal direction, a damping value is to be calculated in orderto calculate the Q value, and generally known damping coefficients andequations for calculating them are as follows:

[0073] Viscous damping coefficient of Couette flow${B_{Couette} = {\mu \frac{A}{g}}},$

[0074] where g represents the thickness of air film and A shows the areaof a sphere;

[0075] Damping coefficient of Stokes flow${B_{Stokes} = {\mu \frac{A}{\delta}}},$

[0076] where δ represents a penetration depth to be defined as${{\delta (\omega)} = \sqrt{\frac{2v}{\omega}}},$

[0077] and A indicates the area of the structure;

[0078] Squeeze damping coefficient${B_{Hagen} = {7.2\quad \mu \quad {l\left( \frac{h}{g} \right)}^{3}}},$

[0079] where g represents the thickness of air film between two plates,l is the superimposed length of the structure, and h indicates theheight of the structure.${{Q\quad {factor}} = {\frac{\sqrt{K_{SYSTEM}M_{SYSTEM}}}{B} = \frac{\sqrt{K_{SYSTEM}M_{SYSTEM}}}{B_{Couette} + B_{Stokes} + B_{Hagen}}}},{{where}\quad \mu}$

[0080] represents the air's absolute viscosity (1.78×10⁻⁵[N·s/m²]).

[0081] The driver has 426 combed electrode structures and 6 foldedspring structures for driving. When the length of the spring is set tobe 192 μm, the spring coefficient is calculated to be 2,684 and thedriving resonance frequency to be 8,299 Hz. The Q factor in atmosphericpressure is calculated considering the size of the whole structure to be3,565. When the driving voltage is set to be V(t)=ν_(d)+ν_(s) cosω_(DRIVE)t=15+3 cos(2π·f_(DRIVE)·t)[V], the driving force and thedriving displacement are calculated as follows.

[0082] Driving force:${F(t)} = {{{4 \cdot ɛ_{0} \cdot N_{DRIVE} \cdot \frac{h}{g} \cdot v_{d} \cdot v_{s} \cdot \cos}\quad \omega_{{DRIVE}^{t}}} = {6.789 \times 10^{- 6}\cos \quad \omega_{{DRIVE}^{t}{\lbrack N\rbrack}}}}$

[0083] Driving displacement:${x(t)} = {{{4 \cdot ɛ_{0} \cdot N_{DRIVE} \cdot \frac{h}{g} \cdot v_{d} \cdot v_{s} \cdot \frac{Q_{DRIVE}}{K_{DRIVE}} \cdot \cos}\quad \omega_{{DRIVE}^{t}}} = {9.017\quad \cos \quad {\omega_{{DRIVE}^{t}}\lbrack{\mu m}\rbrack}}}$

[0084] The detector has 692 combed electrode structures and 6 foldedsprings. When the length of the spring is set to be 157 μm, theresonance frequency of the detector is 8,269 Hz which is marginally lessthan the driving resonance frequency. The Q factor of the detector iscalculated to be 3,765. In this instance, the Coriolis force generatedby an input angular rate and the corresponding displacement arecalculated as follows.

[0085] Coriolis force:$F_{CORIOLIS} = {{2M_{SHUTTLE}{V \cdot \Omega}} = {{2{M_{SHUTTLE} \cdot 4}ɛ_{0}N_{DRIVE}\frac{h}{g}v_{d}v_{s}\frac{Q_{DRIVE}}{K_{DRIVE}}\omega_{DRIVE}\sin \quad {\omega_{{DRIVE}^{t}} \cdot \Omega_{rad}}} = {1.126 \times 10^{- 8}\sin \quad {\omega_{{DRIVE}^{t}} \cdot \Omega_{\deg}}}}}$

[0086] Sensing displacement:${y(t)} = {{\frac{Q_{SENSE}}{K_{SENSE}} \cdot F_{CORIOLIS}} = {0.008\quad \sin \quad {\omega_{{SENSE}^{t}}\left\lbrack {{\mu m}\left( {\text{/}\deg \text{/}\sec} \right)} \right\rbrack}}}$

[0087] Therefore, the mechanical sensitivity of the gyroscope iscalculated to be 0.008[μm(/deg/sec)], and the electrical sensitivity tobe 1.939[fF/deg/sec].

[0088] The process for fabricating the gyroscope according to thepresent invention adopts a single structure layer deep RIE process and asilicon-glass anodic bonding process. By using these, the gyroscopestructure can be fabricated through a single photo etching process, andit can be completed through flip chip bonding with an electrodesubstrate and a PCB.

[0089] The oxidized layer used as a deep RIE etching mask undergoes anundercut in the process of patterning the oxidized layer by an Oxfordetcher, which is reflected to the silicon structure, and finally, thestructure after the deep RIE is fabricated to be narrower than theconventional one by about 2 μm. The thickness of the fabricatedstructure is 44 μm, and the deep RIE is executed for 33 minutes. Afterglass etching for 8 minutes, the structure is completely released fromthe substrate, and in this instance, the gap between the substrate andthe structure is measured to be 37 μm.

[0090] The gyroscope according to the present invention has 426 drivingelectrodes and 692 sensing electrodes, and the driving and sensingelectrodes are mechanically separated from each other through theinertial mass. Also, the gyroscope has a displacement limiter providedin the center of the inertial mass for limiting displacement, and themaximum displacement is 15 μm. Further, a gimbal structure for reducingmechanical interference is provided on the outer portion of the element.

[0091] In order to check the performance of the fabricated devices, asimple driving performance measurement is done using a probe station ofISRC(Inter university Semiconductor Research Center). In the like mannerof the driver, in the case of checking the operation of the sensor,voltage is supplied to measure the resonance frequency and the maximumdisplacement at a resonance frequency, and fabrication and performancemeasuring results are shown roughly in Table 1. TABLE 1 Comb SpringThickness Displacement Frequency width (μm) length (μm) width (μm) (μm)Q (μm) (Hz) Driving Design 5 192 5 44 3565 9.017* 8907 Fabricated 4.3194 4.3 44 2327 4**   6563 Sensing Design 5 165 5 44 3765 0.008/*/s 8863Fabricated 4.3 167 4.3 44 2178 — 6523

[0092] Here, * represents a calculated displacement with applied voltageV(t)=15+3 cos ω_(drive)t [V](bi-directional), and ** shows a measureddisplacement with applied voltage V(t)=7.5 +7.5 cos ω_(drive)t[V](uni-directional).

[0093] When the voltage is supplied to 23 devices to measure theresonance frequencies, 20 devices successfully checked the operation ofthe driver, the operation of the sensor, or the operation of the driverand the sensor. The resonance frequencies range between 5,500 and 6,500Hz, and in particular, they are concentrated in the range between 5,500and 6,000 Hz. Also, in the case of the devices wherein the resonance ofthe driver and the sensor are checked, the difference between the tworesonance frequencies is checked to be uniformly maintained to be about50 Hz. In the case of a device where the resonance in the direction ofeither the driver or the sensor is monitored or in the case of a devicethat has a large difference between the resonance frequencies, thespring is checked to have been broken or adhesion in a single directionto have been generated.

[0094] The displacement at the time of resonance generates a value ofless than the actual designed value as shown in Table 1, since thedisplacement is reduced because of the unidirectional driving, the airdamping is increased and the value Q is reduced because the gap betweenthe substrate and the structure is fabricated narrower than that of thedesigned one, and the gap between the combed electrodes is increasedbecause of footing and undercut of the combed electrode structure.

[0095] Also, the driving voltage is artificially increased to check theefficiency of the displacement limiter. In the case the displacement islimited by the displacement limiter, the driving displacement maximum of15 μm is checked to be maintained in the range of several hundreds Hz,thereby allowing resistance to external noise and obtainment of widebandwidths.

[0096] The present invention provides an electrostatically driven andcapacitively sensed angular rate sensor. By using a single crystalsilicon micromachining technique, a large gyroscope can be fabricated,and accordingly, a gyroscope operable in atmospheric pressure isprovided; and a silicon-glass bonding technique, a silicon etchingtechnique, and flip chip bonding technique are used to fabricate thegyroscope. This represents a very simple process where a gyroscopestructure having etching cavities on its front side is provided througha single photolithographic process, and glass etching is executedwithout an additional photolithographic process. In particular, by usingflip chip bonding, the fabricated gyroscope structure can be directlyintegrated into a circuit without an additional packaging process, andsince the gyroscope can be operable in atmospheric pressure, the presentgyroscope can solve the problems of the vacuum sealing process ofconventional gyroscopes.

[0097] The structure is designed so that it may have as many driving andsensing electrodes as possible and as large an inertial mass as possibleso as to obtain great sensitivity in atmospheric pressure, and it mayhave a large driving operation displacement at high frequencies so as toremove influences caused by external noise. Since the gyroscopestructure has the driving mode and the sensing mode on the same plane,it adopts an operation principle that can remove mechanical interferenceof the conventional gyroscope, and it has a perimeter gimbal structureso as to minimize mechanical interference. Further, a displacementlimiter for limiting driving displacement is mechanically added so as toprovide a gyroscope that has quality sensitivity without additionalfrequency tuning and to design the operation bandwidth of the gyroscopeto be increased.

[0098] The fabricated gyroscope is operable in atmospheric pressure, andis very sensitive in atmospheric pressure.

[0099] While this invention has been described in connection with whatis presently considered to be the most practical and preferredembodiment, it is to be understood that the invention is not limited tothe disclosed embodiments, but, on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

What is claimed is:
 1. A gyroscope comprising: a driving fixed electrodebeing fixed; a driving displacement electrode being opposite to thedriving fixed electrode, and being able to be displaced in a firstdirection; an inertial mass being connected to the driving displacementelectrode, being displaced in the first direction according to the firstdirectional displacement of the driving displacement electrode, andbeing displaced in a second direction when an angular rate is applied; asensing displacement electrode being connected to the inertial mass, andbeing able to be displaced in the second direction according to thesecond directional displacement of the inertial mass; and a sensingfixed electrode being opposite to the sensing displacement electrode andbeing fixed.
 2. The gyroscope of claim 1, wherein the drivingdisplacement electrode is supported by a folded spring movable in thefirst direction, and the sensing displacement electrode is supported bya folded spring movable in the second direction.
 3. The gyroscope ofclaim 1, wherein the driving displacement electrode and the inertialmass can be movable in the second direction and are connected by afolded spring having no fixture shaft, and the sensing displacementelectrode and the inertial mass can be movable in the first directionand are connected by a folded spring having no fixture shaft.
 4. Thegyroscope of claim 1, wherein two sensing displacement electrodes areprovided, one on each of two sides of the inertial mass, and thegyroscope further comprises an edge gimbal for connecting the twosensing displacement electrodes.
 5. The gyroscope of claim 1, wherein acavity is provided in the center of the inertial mass, and the gyroscopefurther comprises a displacement limit shaft provided to the center ofthe cavity and being fixed.
 6. The gyroscope of claim 1, wherein thegyroscope further comprises tuning electrodes, symmetrically formed onboth sides of the sensing displacement electrode, functioning aselectrical springs to vary the resonance frequency of the sensingdisplacement electrode and control sensing sensitivity.
 7. The gyroscopeof claim 6, wherein spaced tooth shapes are respectively formed on thesensing displacement electrode's surface and on the tuning electrode'ssurface facing the sensing displacement electrode, and tooth portions ofthe sensing displacement electrode are facing those of the tuningelectrode, and spaces between the tooth portions of the sensingdisplacement electrode are facing those of the tuning electrode.
 8. Thegyroscope of claim 1, wherein spaced tooth shapes are provided to thefacing sides of the driving fixed electrode and the driving displacementelectrode, and to those of the sensing fixed electrode and the sensingdisplacement electrode, to be interspersed with each other.
 9. A methodfor fabricating a gyroscope comprising: (a) performing anodic bonding ona silicon substrate and a glass substrate; (b) etching and polishing thesilicon substrate to be a predetermined thickness; (c) forming ametallic layer on the silicon substrate; (d) performing photolithographyon the metallic layer and the silicon substrate to form a siliconstructure having a gyroscope pattern; (e) etching the glass substrate,and separating remaining portions of the silicon structure except afixture shaft from the glass substrate to form a gyroscope structure;and (f) performing flip chip bonding on the gyroscope structure toconnect it to an external circuit.
 10. The method of claim 9, whereinthe method further comprises dicing the silicon substrate and the glasssubstrate and separating them into respective elements between (d) and(e).
 11. The method of claim 9, wherein the metallic layer formed in (c)is a double layer of Cr and Au.
 12. The method of claim 9, wherein theglass substrate is etched using an HF solution in (e).
 13. The method ofclaim 12, wherein the depth for etching the glass substrate using the HFsolution is set to be greater than 10 μm so as to minimize air dampingand to operate the gyroscope in atmospheric pressure.
 14. The method ofclaim 9, wherein the silicon substrate is etched using a KOH aqueoussolution of 36 wt. % at 80° C. in (b).